Optical disc drives for reading and/or writing data from/on various types of optical discs such as DVDs (digital versatile discs) have been developed and used extensively nowadays. Recently, as more and more sorts of electronic information get stored on optical discs, the optical disc drives are increasingly required to operate with even higher reliability and come in even handier.
Data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a base material on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are all smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” actually has a physical dimension in the depth direction, too, the term “storage plane” will be replaced herein by another term “storage layer”. Every optical disc has at least one such storage layer. Optionally, a single storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on a recordable optical disc or to write data on such an optical disc, the light beam always needs to maintain a predetermined converging state on a target track on a storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the storage layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position of the light beam is always located on the storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
Read-only optical discs with multiple storage layers have been developed and have become more and more popular these days. Meanwhile, to meet demand for storing data of a huge size, recordable optical discs with multiple storage layers have also been proposed. Such an optical disc with multiple storage layers is called a “multilayer disc”.
To sequentially read data that is stored over multiple storage layers, the operation of jumping the focal point of a light beam from one storage layer to another (which is a so-called “focus jump operation”) needs to be performed. The focus jump can get done by performing a focus control and a tracking control in combination.
For example, Patent Document No. 1 discloses an optical disc drive that performs a focus jump operation. If the target address location is inside of the current address location, the optical disc drive once performs a seek operation until the radial location corresponding to the target address location is reached in the same storage layer, and then performs a focus jump operation to get to the target address location. On the other hand, if the target address location is outside of the current address location, the optical disc drive performs a focus jump operation from the current address location and then performs a seek operation to reach the target address location.
To read and write data from/on a multilayer disc just as intended, however, various influences should be taken into consideration.
In a multilayer disc, to make the reflectances of respective storage layers substantially constant, one of the storage layers to be irradiated with the light beam first should have a high transmittance, thus lowering the reflectance that should be constant. As a result, the optical disc is produced such that the reflectances of the respective storage layers are substantially equal to this low reflectance. Consequently, the signal levels and SNRs of various signals decrease.
Meanwhile, as for areas in each storage layer, the reflectance changes according to a property of the disc between an unrecorded area in which no data has been written yet and a recorded area in which data has already been written. The disc may have the following two types of properties—a property that the reflectance decreases by writing data thereon (as in a DVD-RAM, for example) or a property that the reflectance increases by writing data thereon. This variation in reflectance is normally as much as twice or more. In the vicinity of the boundary between the recorded and unrecorded areas, various types of signals are affected by this reflectance variation. If particularly precise servo is required, the influence of this variation could raise a serious disturbance.
A read-only device (i.e., a player) generates a phase difference tracking error signal (which will be referred to herein as a “phase difference TE signal”) based on the light that has been reflected from a sequence of pits on an optical disc and performs a tracking control using this phase difference TE signal. On a read-only optical disc, data is always stored as pits. That is why as long as the focal point is on the track on the optical disc, the phase difference TE signal can be generated from any area and the tracking control can be performed using it. On top of that, since address information has been added to the data, the optical pickup can be positioned just as intended.
If such a player were loaded with a recordable optical disc with an unrecorded area, the tracking control could not be performed anymore. In the unrecorded area, there is no sequence of marks corresponding to a sequence of pits on a read-only optical disc. That is why if the focal point of a light beam entered the unrecorded area, no phase difference TE signal could be generated and the tracking control could not be performed just as intended.
On the other hand, a device compatible with a recordable optical disc (i.e., a recorder) needs to write data on the unrecorded area of an optical disc, and therefore, performs a tracking control by a push-pull method that involves the presence of a sequence of marks. To perform a tracking control by the push-pull method, a push-pull tracking error signal should be generated from a groove on an optical disc.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2000-251271        